1. Field of the Invention
The present invention relates to communications systems. More particularly, the present invention relates to wireless discrete multitone spread spectrum communications systems.
2. Description of Related Art
Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths and must make highly efficient use of the scarce bandwidth resource for providing quality service to a large population of users. Code Division Multiple Access (CDMA) protocol has been used for wireless communications systems for making efficiently use of limited bandwidths and uses a unique code for distinguishing each user's data signal from other users' data signals. Knowledge of the unique code with which any specific information is transmitted permits separation and reconstruction of each user's message at a receiving end of a communication channel.
The personal wireless access network (PWAN) system described in the referenced Alamouti, Stolarz, et al. patent application uses a form of the CDMA protocol known as discrete multitone spread spectrum (DMT-SS) for providing efficient communications between a base station and a plurality of remote units. In the DMT-SS protocol, a user's data signal is modulated by a set of weighted discrete frequencies or tones that are spreading codes that distribute the data signal over many discrete tones covering a broad range of frequencies. The weights are complex numbers having a real component that is used for modulating the amplitude of a tone and a complex component that is used for modulating the phase of the same tone. Each tone in the weighted-tone set bears the same data signal. Plural users at the transmitting station can use the same tone set for transmitting their data, but each user sharing the tone set has a different set of spreading codes. The weighted-tone set for a particular user is transmitted to the receiving station where it is processed with despreading codes related to the user's spreading codes, for recovering the user's data signal. For each of a plurality of spatially separated antennas at the receiver, the received multitone signals are transformed from time-domain signals to frequency-domain signals. Despreading weights are assigned to each frequency component of the signals received by each antenna element. The values of the despreading weights are combined with the received signals for obtaining an optimized approximation of individual transmitted signals characterized by a particular multitone set and transmitting location.
The PWAN system disclosed in the referenced Alamouti, Stolarz, et al. application has a total of 2560 discrete tones (carriers) that are equally spaced in 8 MHz of available bandwidth in the frequency range of 1850 to 1990 MHz. The frequency spacing between each tone is 3.125 KHz. The total set of tones are numbered consecutively from 0 to 2559, starting from the lowest frequency tone. The tones are used for carrying traffic messages and overhead messages between a base station and a plurality of remote units (RUs). The traffic tones are divided into 32 traffic partitions, with each traffic channel requiring at least one traffic partition of 72 tones.
In addition, the PWAN system uses overhead tones for establishing synchronization and for passing control information between a base station and remote units. A Common Link Channel (CLC) is used by a base station for transmitting control information to a remote unit. A Common Access Channel (CAC) is used for transmitting messages from a remote unit to a base station. There is one grouping of tones assigned to each channel. The overhead channels are used in common by all remote units when control messages are exchanged with a base station.
In the PWAN system, Time Division Duplexing (TDD) is used by a base station and a remote unit for transmitting data and control information in both directions over the same multi-tone frequency channel. Transmission from a base station to a remote unit is called "forward transmission" and transmission from a remote unit to a base station is called "reverse transmission". The time between recurrent transmissions from either a remote unit or a base station is called the TDD period. In every TDD period, there are four consecutive transmission bursts in each direction. Data is transmitted during each burst using multiple tones. A base station and each remote unit must synchronize and conform to the TDD timing structure and both a base station and a remote unit must synchronize to a framing structure. All remote units and base stations are globally synchronized so that all remote units transmit at the same time and then all base stations transmit at the same time. When a remote unit initially powers up, it acquires synchronization from a base station so that it can exchange control and traffic messages within the prescribed TDD time format. A remote unit must also acquire frequency and phase synchronization for the DMT-SS signals so that the remote unit is operating at the same frequency and phase as the base station.
The PWAN system uses a retro directivity principle that relies on the transmit and receive paths at the base station being identical. Since the base station electronics are not identical because of the effects of component tolerances, component drift over temperature and time, and other commonly encountered phenomena, measurements of the transfer functions of the circuitry used for both paths are made. A set of compensating weights are generated and applied to transmitted data so that the forward and reverse paths appear identical at an antenna. The compensation weights provide an additive or more applicable factor making the effect of the despreading weights and the spreading weights the same.
Further, since the PWAN system uses a TDD format, the compensation measurements of the transmit and receive path circuitry are made during the respective idle times of the paths. The time domain duplexing of an airlink results in a 50% duty cycle for utilization of the transmit and receive circuits. Therefore, compensation measurements for the circuitry of a particular path are performed when an airlink does not require its use. Use of the transmit/receive duty cycle of the forward and reverse circuits for making transmit/receive compensation measurements frees system bandwidth and provides much greater measurement flexibility.
FIG. 3 shows a prior-art series-shunt switch 30 that has been used as a transmit/receive switch. An antenna node 31 is connected through a diode 32 to a transmit port TX, and through a quarter-wavelength transmission line 33 to receive port RX. A diode 34 is connected between receive port RX and a signal common. When a bias voltage is applied at V.sub.b, then both diodes 32 and 34 are biased to a low impedance state. The low impedance of diode 34 is transformed into a high impedance at antenna node 31 through quarter-wavelength transmission line 33, thus isolating the RX port from the transmission power applied at the TX port. Switch 30 is unsuitable for allowing testing during idle times of transmit and receive paths because it does not adequately isolate one path from the other during idle times.
FIG. 4 shows a prior art all-shunt diode SP2T switch 40 that has also been used as a transmit/receive switch. An antenna node 41 is connected transmit port TX through a quarter-wavelength transmission line 42, and to a receive port RX through a quarter-wavelength transmission line 43. A diode 44 is connected from the TX port to a signal common and, similarly, a diode 45 is connected from the RX port to the signal common. Capacitors C provide DC isolation between the TX port and the RX port. The antenna node 41 is connected to the TX port when diode 45 is biased into a low-impedance state. The low impedance of diode 45 is transformed into a high impedance at antenna node 41 through quarter-wavelength transmission line 43. When diode 44 is biased into a low-impedance state, antenna node 41 is connected to the RX port. Switch 40 is unsuitable for allowing transmit and receive path testing during path idle times because switch 40 does not adequately isolate one path from the other during path idle times.
What is needed is a transmit/receive switch that allows transmit and receive paths of the PWAN system to be tested during idle times.